The disease is classically a five-day fever of the relapsing type, rarely exhibiting a continuous course. The incubation period is relatively long, at about two weeks. The onset of symptoms is usually sudden, with high fever, severe headache, pain on moving the eyeballs, soreness of the muscles of the legs and back, and frequently hyperaesthesia of the shins. The initial fever is usually followed in a few days by a single, short rise but there may be many relapses between periods without fever. The most constant symptom is pain in the legs. Recovery takes a month or more. Lethal cases are rare, but in a few cases "the persistent fever might lead to heart failure". Aftereffects may include neurasthenia, cardiac disturbances and myalgia.

"Bartonella quintana" is transmitted by contamination of a skin abrasion or louse-bite wound with the faeces of an infected body louse ("Pediculus humanus corporis"). There have also been reports of an infected louse bite passing on the infection.

Most people who are infected develop sickness between five and 15 days after they are bitten. The symptoms may include a sudden fever, chills, headaches, muscle or joint aches, and nausea. A rash may also occur. These symptoms usually continue for two to 9 days, then disappear. This cycle may continue for several weeks if the person is not treated.

The diagnosis of relapsing fever can be made on blood smear as evidenced by the presence of spirochetes. Other spirochete illnesses (Lyme disease, syphilis, leptospirosis) do not show spirochetes on blood smear. Although considered the gold standard, this method lacks sensitivity and has been replaced by PCR in many settings.

It is also called the hematic phase. The most common findings are fever (usually sustained, but with temperature no greater than 102 °F (39 °C)), pale appearance, malaise, painless liver enlargement, jaundice, enlarged lymph nodes, and enlarged spleen. This phase is characterized by severe hemolytic anemia and transient immunosuppression. The case fatality ratios of untreated patients exceeded 40% but reach around 90% when opportunistic infection with "Salmonella spp" occurs. In a recent study, the attack rate was 13.8% (123 cases) and the case-fatality rate was 0.7%.

Other symptoms include a headache, muscle aches, and general abdominal pain. Some studies have suggested a link between Carrion's disease and heart murmurs due to the disease's impact on the circulatory system. In children, symptoms of anorexia, nausea, and vomiting have been investigated as possible symptoms of the disease.

Most of the mortality of Carrion's disease occurs during the acute phase. Studies vary in their estimates of mortality. In one study, mortality has been estimated as low as just 1% in studies of hospitalized patients, to as high as 88% in untreated, unhospitalized patients. In developed countries, where the disease rarely occurs, it is recommended to seek the advice of a specialist in infectious disease when diagnosed. Mortality is often thought to be due to subsequent infections due to the weakened immune symptoms and opportunistic pathogen invasion, or consequences of malnutrition due to weight loss in children. In a study focusing on pediatric and gestational effects of the disease, mortality rates for pregnant women with the acute phase were been estimated at 40% and rates of spontaneous abortion in another 40%.

It is also called the eruptive phase or tissue phase, in which the patients develop a cutaneous rash produced by a proliferation of endothelial cells and is known as "Peruvian warts" or "verruga peruana". Depending on the size and characteristics of the lesions, there are three types: miliary (1–4 mm), nodular or subdermic, and mular (>5mm). Miliary lesions are the most common. The lesions often ulcerate and bleed.

The most common findings are bleeding of verrugas, fever, malaise, arthralgias (joint pain), anorexia, myalgias, pallor, lymphadenopathy, and liver and spleen enlargement.

On microscopic examination, the chronic phase and its rash are produced by angioblastic hyperplasia, or the increased rates and volume of cell growth in the tissues that form blood vessels. This results in a loss of contact between cells and a loss of normal functioning.

The chronic phase is the more common phase. Mortality during the chronic phase is very low.

Trench fever, also known as five-day fever or quintan fever, is the initial manifestation of "B. quintana" infection. Clinical manifestations range from asymptomatic infection to severe illness. Classical presentations include a febrile illness of acute onset, headache, dizziness, and shin pain. Chronic infection manifestations include attacks of fever and aching in some cases and persistent bacteremia in soldiers and homeless people.

"B. henselae" and "B. quintana" can cause bacillary angiomatosis, a vascular proliferative disease involving mainly the skin, and other organs. The disease was first described in human immunodificiency virus (HIV) patients and organ transplant recipients. Severe, progressive and disseminated disease may occur in HIV patients. Differential diagnoses include Kaposi´s sarcoma, pyogenic granuloma, hemangioma, "verruga Peruana", and subcutanous tumors. Lesions can affect bone marrow, liver, spleen, or lymph nodes.

The acute stage of the disease, occurring most often in the spring and summer, begins one to three weeks after infection and lasts for two to four weeks. Clinical signs include a fever, petechiae, bleeding disorders, vasculitis, lymphadenopathy, discharge from the nose and eyes, and edema of the legs and scrotum. There are no outward signs of the subclinical phase. Clinical signs of the chronic phase include weight loss, pale gums due to anemia, bleeding due to thrombocytopenia, vasculitis, lymphadenopathy, dyspnea, coughing, polyuria, polydipsia, lameness, ophthalmic diseases such as retinal hemorrhage and anterior uveitis, and neurological disease. Dogs that are severely affected can die from this disease.

Although people can get ehrlichiosis, dogs do not transmit the bacteria to humans; rather, ticks pass on the "ehrlichia" organism. Clinical signs of human ehrlichiosis include fever, headache, eye pain, and gastrointestinal upset. It is quite similar to Rocky Mountain spotted fever, but rash is not seen in patients.

Cat-scratch disease commonly presents as tender, swollen lymph nodes near the site of the inoculating bite or scratch or on the neck, and is usually limited to one side. This condition is referred to as regional lymphadenopathy and occurs 1–3 weeks after inoculation. Lymphadenopathy in CSD most commonly occurs in the arms, neck, or jaw, but may also occur near the groin or around the ear. A vesicle or an erythematous papule may form at the site of initial infection. Most patients also develop systemic symptoms such as malaise, decreased appetite, and aches. Other associated complaints include headache, chills, muscular pains, joint pains, arthritis, backache, and abdominal pain. It may take 7 to 14 days, or as long as two months, for symptoms to appear. Most cases are benign and self-limiting, but lymphadenopathy may persist for several months after other symptoms disappear. The disease usually resolves spontaneously, with or without treatment, in one month.

In rare situations, CSD can lead to the development of serious neurologic or cardiac sequelae such as meningoencephalitis, encephalopathy, seizures, or endocarditis. Endocarditis associated with "Bartonella" infection has a particularly high mortality. Parinaud's oculoglandular syndrome is the most common ocular manifestation of CSD, and is a granulomatous conjunctivitis with concurrent swelling of the lymph node near the ear. Optic neuritis or neuroretinitis is one of the atypical presentations.

Immunocompromised patients are susceptible to other conditions associated with "B. henselae" and "B. quintana", such as bacillary angiomatosis or bacillary peliosis. Bacillary angiomatosis is primarily a vascular skin lesion that may extend to bone or be present in other areas of the body. In the typical scenario, the patient has HIV or another cause of severe immune dysfunction. Bacillary peliosis is caused by "B. henselae" that most often affects patients with HIV and other conditions causing severe immune compromise. The liver and spleen are primarily affected, with findings of blood-filled cystic spaces on pathology. In 2015 a Toledo, Ohio woman lost eyesight in an eye after a cat licked it.

Ehrlichiosis (; also known as canine rickettsiosis, canine hemorrhagic fever, canine typhus, tracker dog disease, and tropical canine pancytopenia is a tick-borne disease of dogs usually caused by the organism "Ehrlichia canis". "Ehrlichia canis" is the pathogen of animals. Humans can become infected by "E. canis" and other species after tick exposure. German Shepherd Dogs are thought to be susceptible to a particularly severe form of the disease, other breeds generally have milder clinical signs. Cats can also be infected.

Cat-scratch disease (CSD) is a common and usually benign infectious disease caused by the bacterium "Bartonella henselae". It is most commonly found in children following a scratch or bite from a cat within about one to two weeks.

Affected individuals typically develop symptoms including high fevers, shaking, chills, fatigue, headaches, vomiting, and general illness within 48 hours of the initial infection. The erythematous skin lesion enlarges rapidly and has a sharply demarcated, raised edge. It appears as a red, swollen, warm, and painful rash, similar in consistency to an orange peel. More severe infections can result in vesicles (pox or insect bite-like marks), blisters, and petechiae (small purple or red spots), with possible skin necrosis (death). Lymph nodes may be swollen, and lymphedema may occur. Occasionally, a red streak extending to the lymph node can be seen.

The infection may occur on any part of the skin, including the face, arms, fingers, legs, and toes; it tends to favour the extremities. Fat tissue and facial areas, typically around the eyes, ears, and cheeks, are most susceptible to infection. Repeated infection of the extremities can lead to chronic swelling (lymphangitis).

Common signs and symptoms include:

- sore throat

- red, swollen tonsils

- pain when swallowing

- high temperature (fever)

- headache

- tiredness

- chills

- a general sense of feeling unwell (malaise)

- white pus-filled spots on the tonsils

- swollen lymph nodes (glands) in the neck

- pain in the ears or neck

- weight loss

- difficulty ingesting and swallowing meal/liquid intake

- difficulty sleeping

Less common symptoms include:

- nausea

- fatigue

- stomach ache

- vomiting

- furry tongue

- bad breath (halitosis)

- voice changes

- difficulty opening the mouth (trismus)

- loss of appetite

- Anxiety/fear of choking

In cases of acute tonsillitis, the surface of the tonsil may be bright red and with visible white areas or streaks of pus.

Tonsilloliths occur in up to 10% of the population frequently due to episodes of tonsillitis.

Erysipelas is an acute infection typically with a skin rash, usually on any of the legs and toes, face, arms, and fingers. It is an infection of the upper dermis and superficial lymphatics, usually caused by beta-hemolytic group A "Streptococcus" bacteria on scratches or otherwise infected areas. Erysipelas is more superficial than cellulitis, and is typically more raised and demarcated. The term is from Greek ἐρυσίπελας, meaning "red skin".

It resembles bacterial sepsis and can occur after initiation of antibacterials, such as mild silver protein, penicillin or tetracycline, for the treatment of louse-borne relapsing fever (80–90% of patients) and in tick-borne relapsing fever (30–40%). It usually manifests within a few hours of the first dose of antibiotic as fever, chills, rigor, hypotension, headache, tachycardia, hyperventilation, vasodilation with flushing, myalgia (muscle pain), exacerbation of skin lesions and anxiety. The intensity of the reaction indicates the severity of inflammation. Reaction commonly occurs within two hours of drug administration, but is usually self-limiting. It is observed in 50% of patients with primary syphilis and about 90% of patients with secondary syphilis.

Tonsillitis is inflammation of the tonsils, typically of rapid onset. It is a type of pharyngitis. Symptoms may include sore throat, fever, enlargement of the tonsils, trouble swallowing, and large lymph nodes around the neck. Complications include peritonsillar abscess.

Tonsillitis is most commonly caused by a viral infection, with about 5% to 40% of cases caused by a bacterial infection. When caused by the bacterium group A streptococcus, it is referred to as strep throat. Rarely bacteria such as "Neisseria gonorrhoeae", "Corynebacterium diphtheriae", or "Haemophilus influenzae" may be the cause. Typically the infection is spread between people through the air. A scoring system, such as the Centor score, may help separate possible causes. Confirmation may be by a throat swab or rapid strep test.

Treatment efforts involve improving symptoms and decreasing complications. Paracetamol (acetaminophen) and ibuprofen may be used to help with pain. If strep throat is present the antibiotic penicillin by mouth is generally recommended. In those who are allergic to penicillin, cephalosporins or macrolides may be used. In children with frequent episodes of tonsillitis, tonsillectomy modestly decreases the risk of future episodes.

About 7.5% of people have a sore throat in any three-month period and 2% of people visit a doctor for tonsillitis each year. It is most common in school aged children and typically occurs in the fall and winter months. The majority of people recover with or without medication. In 40% of people, symptoms resolve within three days, and in 80% symptoms resolve within one week, regardless of if streptococcus is present. Antibiotics decrease symptom duration by approximately 16 hours.

Parinaud's oculoglandular syndrome is the combination of granulomatous conjunctivitis in one eye, and swollen lymph nodes in front of the ear on the same side. Most cases are caused by cat-scratch disease, although it is an unusual feature of this condition. Occasionally it may be caused by other infections.

It should not be confused with the neurological syndrome caused by a lesion in the midbrain which is also known as Parinaud's syndrome. Both were named after the same person, Henri Parinaud.

Causes include:

- "Bartonella henselae"

- "Francisella tularensis"

- herpes simplex virus type 1

- "Paracoccidioides brasiliensis"

An emerging infectious disease (EID) is an infectious disease whose incidence has increased in the past 20 years and could increase in the near future. Emerging infections account for at least 12% of all human pathogens. EIDs are caused by newly identified species or strains (e.g. Severe acute respiratory syndrome, HIV/AIDS) that may have evolved from a known infection (e.g. influenza) or spread to a new population (e.g. West Nile fever) or to an area undergoing ecologic transformation (e.g. Lyme disease), or be "reemerging" infections, like drug resistant tuberculosis. Nosocomial (hospital-acquired) infections, such as methicillin-resistant Staphylococcus aureus are emerging in hospitals, and extremely problematic in that they are resistant to many antibiotics. Of growing concern are adverse synergistic interactions between emerging diseases and other infectious and non-infectious conditions leading to the development of novel syndemics. Many emerging diseases are zoonotic - an animal reservoir incubates the organism, with only occasional transmission into human populations.

The Jarisch–Herxheimer reaction is traditionally associated with antimicrobial treatment of syphilis. The reaction is also seen in the other diseases caused by spirochetes: Lyme disease, relapsing fever, and leptospirosis. There have been case reports of the Jarisch-Herxheimer reaction accompanying treatment of other infections, including Q fever, bartonellosis, brucellosis, trichinellosis, and African trypanosomiasis.

Bacillary angiomatosis (BA) is a form of angiomatosis associated with bacteria of the "Bartonella" genus.

Paravaccinia virus presents itself with blisters, nodules, or lesions about 4 mm in diameter, typically in the area that has made contact with livestock that is infected with bovine papular stomatitis. Lesions may begin forming as late as three weeks after contact has been made with an infected animal. In rare cases, lesions may be seen systemic. General signs of infection are also common, such as fever and fatigue.

Infected livestock may present with blisters or lesions on their udders or snout. Often, however, infected livestock show little to no symptoms.

Zoonoses are infectious diseases of animals (usually vertebrates) that can naturally be transmitted to humans.

Major modern diseases such as Ebola virus disease and salmonellosis are zoonoses. HIV was a zoonotic disease transmitted to humans in the early part of the 20th century, though it has now evolved to a separate human-only disease. Most strains of influenza that infect humans are human diseases, although many strains of swine and bird flu are zoonoses; these viruses occasionally recombine with human strains of the flu and can cause pandemics such as the 1918 Spanish flu or the 2009 swine flu. "Taenia solium" infection is one of the neglected tropical diseases with public health and veterinary concern in endemic regions. Zoonoses can be caused by a range of disease pathogens such as viruses, bacteria, fungi and parasites; of 1,415 pathogens known to infect humans, 61% were zoonotic. Most human diseases originated in animals; however, only diseases that routinely involve animal to human transmission, like rabies, are considered direct zoonosis.

Zoonoses have different modes of transmission. In direct zoonosis the disease is directly transmitted from animals to humans through media such as air (influenza) or through bites and saliva (rabies). In contrast, transmission can also occur via an intermediate species (referred to as a vector), which carry the disease pathogen without getting infected. When humans infect animals, it is called reverse zoonosis or anthroponosis. The term is from Greek: ζῷον "zoon" "animal" and νόσος "nosos" "sickness".

Zoonotic transmission can occur in any context in which there is companionistic (pets), economic (farming, etc.), predatory (hunting, butchering or consuming wild game) or research contact with or consumption of animals, animal products, or animal derivatives (vaccines, etc.).

Paravaccinia virus is a viral infection of the Parapoxvirus genus of viruses. Human can contract the virus from contact with livestock infected with Bovine papular stomatitis and is common with ranchers, milkers, and veterinarians. Infection will present with fever, fatigue, and lesion on the skin.